Type in the object name to get its coordinates from Sesame;
or get coordinates from the map view;
or type the coordinates in manually, pressing enter to set the coordinates.
You can paste both RA and dec into one coordinate box and it will be interpreted correctly.
A single number is interpreted as a decimal coordinate, a pair of numbers as a pair of decimal coordinates, and anything else as a sexagesimal representation.
If you are browsing around the sky and you want to know where you ended up, click on "identify objects" to identify interesting objects in the field of view (max 1°).
For solar system objects, click on "advanced options...". Coordinates are obtained from the Miriade service.
If you overlay an instrument field of view, the observatory the instrument is at will automatically be selected.
Type the name of a telescope or observatory to find its coordinates;
or select it on the map;
or (if your device and browser support geolocation) use "set to my location" or "set to nearest observatory";
or enter the coordinates manually, pressing enter to define a new observatory;
If specifying coordinates manually, the time zone must be given as a tz database code.
or double click on the map to select a location. Its elevation and time zone will be automatically determined. This works for points on land only.
Observatories from where all your objects reach airmass<1.4 are shown in blue; observatories from where any of your objects cannot be seen are shown in grey.
The full list of defined observatories can be viewed here
"show night" shows which parts of Earth are in darkness right now (accurate to about 10 minutes in time, 200km in distance)
"show light pollution" overlays the estimated sky brightness from the World Atlas of the Night Sky Brightness
Date
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Date
You can also move by ± 1 day using the keyboard left and right arrows.
optimal will find the day of the year on which the combined altitude of the objects at local midnight is maximised. It will do nothing if no objects rise.
When moving by ± 1 month, surprising results may occur. For example, if you are on March 31, and you go forward one month, the date calculated is April 31, which does not exist and is interpreted instead as May 1.
Daily chart[?]
Daily chart
This chart shows the altitude with time of the objects and the Moon on the selected day.
Grid lines mark hours along the x-axis, and intervals of 5° on the y-axis.
The solid blue line represents the object and the red dashed line represents the Moon.
Blue shading represents the sky brightness due to the Moon. The density of the shading is proportional to the fractional illumination of the Moon.
If there is a change of time during the night, it is marked on the chart, but all local times still reflect the time at the start of the night.
Twilights may look very strange in the Antarctic. This is because Google's time zone resolver sets everywhere on the continent to UTC+12, which means that civil midnight can be many hours away from solar midnight.
Annual chart
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Annual chart
This chart shows the altitude of the object at local midnight throughout the year.
Grid lines mark months along the x-axis, and intervals of 5° on the y-axis.
The solid blue line represents the object and the red dashed line represents the Moon.
The grey shaded regions show the length of civil and astronomical night.
A light red dashed vertical line indicates the date shown in the daily chart.
Click on the chart to replot the daily chart for that date.
In polar regions, the altitude at local midnight will show discontinuities. This is because the definition of local midnight changes: normally half way between sunset and sunrise, it is set to 00:00 local time if the sun doesn't rise, and undefined if the sun doesn't set.
Data
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Data
The tables in this section summarise the information presented visually above.
They include details of the objects and observing location; solar events; and moon position.
Sunset and sunrise times are corrected for refraction and the altitude of the observing location.
Lunar calculations are corrected for refraction and altitude, and are in the topocentric frame. They are slightly simplified, omitting some smaller periodic terms, but should be accurate to within a couple of minutes.
'Get extended data' gives an hour-by-hour breakdown of object and site details: altitude, azimuth, airmass, lunar distance, times, and heliocentric radial velocity correction (with the parts due to Earth's rotation and orbit indicated). Subtract these corrections from an observed velocity to apply them.